Following a link below, I came across this page explaining the conversion of the original LNER A1 Pacifics to the A3 class, of which the Flying Scotsman is the last survivor. Interesting that they were converted from right to left hand drive in the 1950s. An excellent website for London & North Eastern Railway information:

http://www.winwaed.com/rail/LNER/locos/A/a1a3a10.shtml